Development of Digestive System of Animals
The digestive tube consists of an internal tube of endoderm which is the primary tissue that becomes the epithelial lining and an investing layer of splanchnic mesoderm that specializes into the connective tissue, muscle and peritoneum. The primitive gut is made up of three portions, the foregut, midgut and hindgut.
The cranial part of the foregut forms the oesophagus and the caudal part of the foregut forms the descending duodenum. The midgut develops into ascending duodenum, jejunum, ileum, caecum, ascending and transverse colon. The hindgut differentiates into decending colon and rectum.
DEVELOPMENT OF ESOPHAGUS
From the foregut, just behind the pharynx, the oesophagus develops which maintains its tubular shape throughout the life. The musculature in the oesophagus is comprised of smooth muscle that comes from splanchnopleurae of the foregut.
The distribution of skeletal muscle may be from mesenchyme present in the area of bronchial arches (I – VI) and migrates to the nearby foregut wall.
DEVELOPMENT OF MONOGASTRIC STOMACH
The 1st indication of the stomach appears as dilation in the foregut. The dilation enlarges but is not uniform and most of the enlargement is dorsal.
Once the dorsal enlargement is over, the stomach undergoes a 90ºrotation along cranio-caudal axis. Thus the dorsal enlargement rotates forward the left side of the embryo.
Hence the stomach lies with its dorsal part on the left and ventral part on the right. The cranial part of the dorsal enlargement of the stomach grows more than the caudal part forming the fundus.
The boundry of dorsal enlargement becomes greater curvature and of the ventral part is lesser curvature.
DEVELOPMENT OF RUMINANT STOMACH
Monogastric days reached in 33 days and 90o rotation is completed by this time. The area corresponding to fundus becomes rumen. Other differential growth area appears along the greater curvature of embryo stomach just caudal and ventral to the forming rumen. This area is the reticulum.
A third area appears along the lesser curvature. This is the omasum. The abomasum forms from the pyloric region. By 40 days, further differentiation occurs by two cranially directed outgrowths from fundus. One is dorsal and the other is ventral (dorsal and ventral sacs).
By 43 days, rumen is growing caudo-dorsally and forward to the right. This results in reversal positions of the dorsal and ventral sacs. The sacs become directed caudally and the dorsal sac is dorsal to the ventral sac. Now the rumen is on the left side of the abdominal cavity, followed by reticulum, omasum and abomasum towards right.
When the rumen enlarges, it forces the reticulum cranially and ventrally. As the rumen grows, it begins to encroach toward the right. This encroachment pushes the omasum and abomasum ventrally.
Once the adult positions are reached (14 weeks of gestation) the growth of the rumen slows. By birth rumen is 1 1/2 size of abomasum. After birth rumen enlarges till 8 weeks and by 12 weeks, becomes twice the size of abomasum. In adults around 1- ½ yrs of age the size depends on roughage diet.
DEVELOPMENT OF INTESTINE
The intestine is a simple tube beginning in the stomach and ending in the cloaca.The narrow caudal part of the foregut forms the descending duodenum.
The midgut develops into ascending duodenum, jejunum, ileum, caecum, ascending and transverse colon. The hindgut differentiates into decending colon and rectum.
Around 20 days, or even earlier, the midgut grows in length farther than it could be accommodated in abdominal cavity.
The midgut looses connection with the yolksac and at this stage, it is in the form of a loop. This loop has a cranial and a caudal limbs and it undergoes an anti-clockwise rotation. Due to this, the cranial limb goes to the right and behind.
The caudal limb is taken forwards and to the left. Now the gut begins to elongate rapidly and the loop herniates into the umbilical cord. Then cranial and caudal limbs undergo 180orotation.
The original cranial limb of the intestinal loop forms the ascending duodenum, jejunum and ileum. The caudal limb of the loop forms the ceacum and the first part of the colon.
The hindgut forms the terminal part of the colon. The caudal end of the hindgut forms the cloaca.
The subdivision of the cloaca results in the rectum. The endodermal lining obliterates the lumen of the gut in the early stages which later on gets canalized and the lumen gets restored. The lining gets villi throughout initially but later the large intestine loses the villi.
Anomalies of development of intestine
- Stenosis or Atresia of oesophagus – absence of canalization.
- Situs inversus – organs found in exactly opposite situation.
- Umbilical fistula – persistent yolk sac opening to exterior.
- Umbilical hernia – failure of withdrawal of intestine.
- Meckle’s direrticulum of ileum – Persistence of proximal part of yolk stalk.
- Imperforate anus – failure of rupture of the anal membrane.
DEVELOPMENT OF LIVER
Liver is an endodermal derivative. Its primordium lies between the pericardial cavity and attaching yolk stalk. Here the floor of the future duodenum continues to give rise to definite sacculations named hepatic diverticulum. This consists of a cranial portion which will differentiate into the glandular tissue and its bile ducts.
The caudal portion will become the gall bladder and cystic duct. The hepatic diverticulum forces its way to splanchnic mesoderm which forms the primitive diaphragm, the septum transversum.
A little later, the region of the septum occupied by the liver becomes drawn out as ventral mesentery and the final relation to the liver is more related to the mesentery than the diaphragm.
The cranial portion of the hepatic diverticulum buds off epithelial cords which invade the septum transversum and continue to proliferate into a spongy work.
The paired vitelline veins flanking the gut and send the branches into the region of proliferation. The result is a mutual, intimate intergrowth of tortuous liver cords and sinusoidal channels.
The diverticulum in the meanwhile elongates and differentiates into duct system. The main portion of the diverticulum forms the hepatic duct and ductus choledochus.
From the hepatic duct, large intra hepatic ducts buds off and from these small inter lobular ducts arise. The system of branching of vitelline veins are responsible for creation of hepatic lobules from the parenchyma.
The gall bladder and cystic duct develop from caudal portion of hepatic diverticulum. The septum transversum furnishes the peritoneal covering and the connective tissue framework.
DEVELOPMENT OF PANCREAS
It is an endodermal derivative. The outpocketings from the endodermal lining of the gut are the indications of future pancreas. These buds arise on the opposite sides of duodenum, one on the dorsal side and the other on the ventral side.
The dorsal bud lies in front of the hepatic diverticulum. The ventral bud lies in the angle between the gut and the hepatic diverticulum.
Unequal growth of duodenal wall shifts the bile duct dorsally and the ventral primordium of the pancreas is shifted dorsal and is brought nearer to the dorsal primordium. During futher development two primordia fuse.
By proliferation, the duct system develop into acini, some epithelial buds loose connection with ducts and develop as islets.
DEVELOPMENT OF SPLEEN
The spleen is a mesodermal derivative. It is developed in the dorsal mesogastrium. At first an accumulation of mesenchymal cells is seen just beneath the surface of the peritoneal epithelium. This mass increases in size and projects above the omental surface as several hillocks which soon merge.
The part of the dorsal mesogastrium which is attached to the spleen becomes reduced to narrow band called the gastrosplenic omentum. The mesenchyme is soon vascularised and the capsule, trabeculae and pulp cords are differentiated.
